The key reactive sites in the selective catalytic reduction (SCR) of NO using ammonia (NH3–SCR) are the Cu­(II)/Cu­(I) ammine complexes encapsulated within zeolite catalysts. The reaction intermediate in the important CuI → CuII reoxidation step has been proposed as the dicopper species. However, obtaining the definitive spectroscopic probe for the dicopper intermediate under NH3–SCR remains a quite challenging subject. Here we report spectroscopic evidence of the efficient generation of the dicopper intermediate in NH3–SCR over Cu-ion-exchanged zeolites. The state of the Cu ion under the NH3–SCR condition was analyzed using fiber-optic UV–vis–NIR spectroscopy; a distinct spectroscopic fingerprint of the (μ–η2:η2-peroxo)­dicopper­(II) intermediate was detected. Our spectroscopic assignment was rationalized by density functional theory calculations. The intensity of the spectroscopic fingerprints of dimeric copper was much higher than expected from previous reports, and it was also found that such a spectroscopic probe is observable only under the NH3–SCR conditions. This finding strongly supports the previously suggested Cu dynamic contributions to NH3–SCR catalysis.